Field--The invention is in the field of high brightness laser systems which use a low energy master oscillator laser beam and a phase conjugated laser amplifier to amplify the oscillator laser beam. Specifically, the invention is cascaded double conjugated laser amplifiers wherein a switched Pockels cell and a polarizer are associated with each of the double conjugate laser amplifiers with the Pockels cell controlling whether the oscillator beam is transmitted directly through the polarizer along the oscillator beam axis or is reflected off the polarizer into and retroreflected from one of the amplifiers and back through the polarizer into and retroreflected from the second amplifier and reflected off the polarizer in the exact same direction as the original oscillator beam as a twice conjugated and amplified laser system output beam.
Prior Art--A prior art phase conjugated laser amplifier is known which is comprised of a laser oscillator that transmits input beam pulses through a thin film polarizer into a single phase conjugated power amplifier comprised of an amplifier, a quarter-wave plate, and a phase conjugated mirror. The amplifier amplifies the beam pulses once on the first pass therethrough. The quarter-wave plate turns the polarization of the once amplified beam from being linear to circular prior to entering the phase conjugate mirror. The pulses are retroreflected from the mirror back through the quarter-wave plate which turns the polarization from being circular to linear but rotated 90.degree. from the original polarization and is amplified a second time as the pulses are passed through the amplifier. The twice amplified output beam pulse exits the laser system at an oblique angle from the direction of the oscillator input beam pulse after being reflected off the thin film polarizer. This change in direction for the output beam from the input beam can be undesirable. Further, the output beam direction is sensitive to any motion of the polarizer even though the amplifier itself provides distortion free amplification and an output direction stable against movements or vibrations of the amplifier.
The present invention solves the above noted problems of the prior art output beam pulse reflecting off the thin film polarizer at an oblique angle by having each of the double conjugate amplifiers conjugate the beam two times, i.e. conjugated once in each of two singly conjugated amplifiers, which form the double conjugate amplifiers. A thin film polarizer positioned between the amplifiers reflects the oscillator input beam off the polarizer into a first amplifier, passes the retroreflected beam back through the polarizer into a second amplifier, and reflects the retroreflected beam back from the second amplifier off the polarizer again. The output beam comes out of each double conjugate amplifier in exactly the same direction as the original input beam. The output beam direction stays constant even if the polarizer is not stabilized such as when it is mounted on a moving military vehicle.